This invention relates generally to optical fibers. More specifically, the invention relates to a tightly-coiled optical fiber doped with rare earth elements.
Optical fibers are used extensively in a variety of optical applications, such as telecommunication, fiber optic sensors, audio signal transmission, and various other applications requiring optical waveguides. One reason for the optical fiber's widespread use is its ability to efficiently conduct light by confining the optical energy into the optical fiber. If the optical fiber is bent beyond a certain minimum bend radius, however, this efficiency suffers due to bend loss. Bend loss thus restricts optical fiber applications requiring small bend radii such as tight winding of the optical fiber.
Bend loss is especially restricting in applications requiring high optical powers. For instance, relevant telecommunication standards, prescribe approximately 1.75 centimeters (cm) as the minimum bend radius for a standard fiber with a cladding diameter of 125 microns. For higher powers and energies large-mode-area (LMA) fibers are frequently used. LMA fibers provide large mode areas by the lowest numerical aperture (NA) core waveguides. Because a low NA typically correlates to higher bend loss, the implied bend losses for LMA fibers are relatively large. Thus, in very high power devices, the bend radius of typical fluoroacrylate-clad fibers can be roughly 5 cm, and for all-glass structures, it could be as large as 20 cm. It is further assumed that bending the core of a fiber effectively squeezes the modal intensities in the outward direction. In an active fiber, such bending would result in an uneven gain between the centrosymmetric mode and the distal modes. These restrictions on the tight bending of active optical fiber have prevented the use of coiled active optical fiber in compact applications.
It is therefore desirable to provide systems and devices that allow an active optical fiber to be coiled in a tight radius without suffering significant bend loss.